With the discovery of hydrocarbon-producing fields located at ever increasing water depths, the use of rigid structures fixed on the sea bed, intended for installation of production equipment, has become more complex by the day. In some cases, owing to the particular conditions of the region where the hydrocarbon producing field is located, their use has become unviable.
In more recent times, floating structures represent an alternative that is used more and more frequently for the installation of production equipment, as they normally offer a lower-cost option compared to fixed structures.
Petroleum production in deep water makes extensive use of riser pipes, also known as risers. This tubing, which may be flexible or rigid, collects the oil produced by the subsea well, and conveys it to a floating unit, and then to tankers or directly to onshore installations. As the floating structures move, the risers are subjected to cyclic flexure that causes their radii of curvature to vary, which causes fatigue in the region of transfer of shearing forces to the floating unit.
In this scenario, devices known as tensioners are used for protecting flexible risers and umbilicals from being bent at radii less than their minimum permissible radii. The tensioners achieve this by increasing the flexural rigidity in a particular section of the riser. The majority of tensioners are in the form of truncated-cone sections, and are made of elastomeric materials.
The tensioners may be fixed to the floating units in various ways, such as by means of devices called bell mouths, fastened by means of flanges, inserted in conical seatings or inserted in cylindrical devices that have some form of fixation.
The tensioners of the prior art must be dimensioned in such a way as to prevent fatigue of the flexible risers for the design life. The increase in working depth of floating units implies an increase in the tensile loads on the flexible risers. Also, with the movement of the vessel and of the pipes due to currents, there is an increase of the loads from shearing forces and bending moment in the system for fixing the pipes to the platform.
In this scenario of deep-water oil exploration, the increased dimensions and the higher loadings require the use of fixing structures of large dimensions for the tensioners. This makes their distribution and allocation difficult in new floating units. Furthermore, this increase in dimensions makes handling of the tensioners more and more difficult. This increases the offshore working time for installation and dismantling of the flexible risers. The total cost of installation of the floating unit increases not only through the unit cost of each tensioner, but also through the working time of the ships and teams employed for installation of the flexible risers.
In addition to the problem of the increased size and weight of the tensioners and their fixing structures, it is known that, during the basic engineering phase of the design of a platform, it is necessary for the designers to have information about the diameters and top and azimuth angles of all the risers that will be supported by the platform. However, at present, the data are still not available. Estimates are made based on similar platforms and the data obtained are employed in the design. When the basic arrangement with which the platform will work is finally released, there are often discrepancies relative to the initial estimates. As an example, there have already been cases where platforms had to undergo modifications for adaptation to the new scenario, including returning to dry dock, which delays the start of operation and causes enormous losses.
In an attempt to solve the aforementioned problems, document PI0803358-7 discloses a symmetrical tensioner with swivel joint, which comprises a lower protective cone, an upper protective cone, a swivel joint, a spherical bearing and a centralizer. Since it comprises a certain degree of mobility owing to the presence of the swivel joint, the symmetrical tensioner with swivel joint of document PI0803358-7 attenuates the bending moment transmitted to the structure for fixing the tensioner to the platform. In this way, the symmetrical tensioner with swivel joint of document PI0803358-7 may have smaller dimensions than a conventional tensioner of the same loading capacity, which facilitates its fabrication, transport, handling, positioning and installation in a production unit.
However, the symmetrical tensioner with swivel joint described in document PI0803358-7 does not satisfactorily solve the problems of the prior art. This is because, due to the limitation of movement inherent in a system with swivel joint, it cannot completely alleviate the bending moment stresses and shearing force. In addition, for the same reason, the symmetrical tensioner with swivel joint described in document PI0803358-7 does not allow sufficient compensation if there are significant changes in the design in relation to the top and azimuth angles of the pipes, depending on the arrangement of wells and components on the sea bed. Moreover, the symmetrical tensioner with swivel joint described in document PI0803358-7 is of complex assembly, installation and maintenance, due to the manner in which the components are interconnected.
Other alternatives in the prior art also do not satisfactorily solve the aforementioned problems. Document WO2015104560, for example, discloses a segmented tensioner formed from two or more segments, each segment comprising a male end and a female end for engaging with the adjacent segment. The interface between the segments provides a small deflection between them, sufficient to reduce the stresses in the tensioner. However, the solution presented in document WO2015104560 is very complex in assembly, installation and maintenance mainly owing to the large number of moving parts.
In addition, it is known that the method for installing flexible risers (known as pull-in) employed conventionally in the prior art requires an excessive expenditure of time, with employment of a diver and mobilization of support ships. Unfavourable sea conditions may even require suspension of the installation work, since at present there are no methods of installation that are independent of the weather conditions. This results in delays in the production of the well associated with the riser.
As will be presented in more detail below, the present disclosure aims to solve the problems of the prior art described above in a practical and efficient manner and at low cost.